82562EZ(EX)/82547GI(EI) Dual Footprint Design Guide
Standard capacitor loads used by crystal manufacturers include 16 pF, 18 pF and 20 pF. Any of
these values will generally operate with the controller. However, a difference of several picofarads
between the calibrated load and the actual load will pull the oscillator slightly off frequency.
Note:
C1 and C2 may vary by as much as 5% (approximately 1 pF) from their nominal values.
3.1.1.7
Shunt Capacitance
The shunt capacitance parameter is relatively unimportant compared to load capacitance. Shunt
capacitance represents the effect of the crystal’s mechanical holder and contacts. The shunt
capacitance should equal a maximum of 6 pF (7 pF is also acceptable).
3.1.1.8
Equivalent Series Resistance
Equivalent Series Resistance (ESR) is the real component of the crystal’s impedance at the
calibration frequency, which the inverting amplifier’s loop gain must overcome. ESR varies
inversely with frequency for a given crystal family. The lower the ESR, the faster the crystal starts
up. Use crystals with an ESR value of 50
Ω
or better.
Note:
Check the specific controller documentation carefully; some devices may have tighter ESR
requirements. For example, Intel recommends that 82547GI(EI) devices use crystals with an ESR
value of 10
Ω
or less.
3.1.1.9
Drive Level
Drive level refers to power dissipation in use. The allowable drive level for a Surface Mounted
Technology (SMT) crystal is less than its through-hole counterpart, because surface mount crystals
are typically made from narrow, rectangular AT strips, rather than circular AT quartz blanks.
Some crystal data sheets list crystals with a maximum drive level of 1 mW. However, Intel®
Ethernet controllers drive crystals to a level less than the suggested 0.5 mW value. This parameter
does not have much value for on-chip oscillator use.
3.1.1.10
Aging
Aging is a permanent change in frequency (and resistance) occurring over time. This parameter is
most important in its first year because new crystals age faster than old crystals. Use crystals with a
maximum of ±5 ppm per year aging.
3.1.2
Reference Crystal
The normal tolerances of the discrete crystal components can contribute to small frequency offsets
with respect to the target center frequency. To minimize the risk of tolerance-caused frequency
offsets causing a small percentage of production line units to be outside of the acceptable frequency
range, it is important to account for those shifts while empirically determining the proper values for
the discrete loading capacitors, C1 and C2.
Even with a perfect support circuit, most crystals will oscillate slightly higher or slightly lower than
the exact center of the target frequency. Therefore, frequency measurements (which determine the
correct value for C1 and C2) should be performed with an ideal reference crystal. When the
capacitive load is exactly equal to the crystal’s load rating, an ideal reference crystal will be
perfectly centered at the desired target frequency.
10
INTEL [ INTEL CORPORATION ]
